The set up

The set up
5.46mm jet delivering 0.68 l/s to the pelton which is rotating at 900 rpm and generating 135 watts into the grid.

Thursday, 7 July 2016

Rotor packing

The weather has been cool and cloudy here in Wales for what seems far too long. There has been rainfall too, more than usual for this time of year, but none of this has stopped flow to the turbine steadily diminishing.  Today was a day for reducing the size of nozzle to keep flow delivered matched to what is available.

Doing this gave an opportunity.  I'm working in previously uncharted territory trying to make the most of these smaller summer-time flows whilst using a non-MPPT inverter**, and today's nozzle change gave scope to experiment.  

In previous diary posts, I've outlined what the aim is, - to keep pelton speed up in order to have it operate near its optimum rpm where most energy is extracted from the head and flow available. Keeping speed up can be achieved by making the rotor stand off from the stator coils by placing packing washers on the shaft. But the method is one of finding by experiment how much packing produces the best output: too little makes rpm and power output low; too much makes rpm better but reduces alternator efficiency so the end result is just as bad.  Somewhere in the middle lies a best compromise and the challenge is to find it. These pictures show how it can be done:

For these photos, the camera was carefully aligned with the back of the turbine casing so the amount of rotor stand off would be captured along with the rpm on the tachometer.  As can be seen, 8 mm of stand off produced most watts at a speed of 876 rpm, so that was how I left it, but I can't help feeling a tad more, say 10 mm taking rpm just into the 900's, could be better.  

That'll be a job for tomorrow perhaps...***

** It is worth stressing that I am only experimenting with rotor packing because this inverter does NOT use Maximum Power Point Tracking (MPPT) to optimise the power fed to it.  Inverters that use MPPT should find optimum speed automatically and with greater accuracy. They should therefore always give maximum possible power so long as voltage is within the range specified for MPPT tracking.  My own installation probably proved this last year: then I was using an MPPT inverter and with the turbine set up precisely as it was today except for having ceramic bearings and a reduced core stator, the output to grid was 299 watts.  Allowing 10 watts extra output for the decreased rolling resistance of ceramics compared to standard greased bearings, this was 14 watts more than the best I could achieve today (275 watts) with experimental rotor packing.

***... and indeed it was a job done later: an extra 1.75 mm (one of EcoInnovation's special packing washers, specially made for the job) was added to the existing packing of two rubber O-rings. It meant that the 'knob' only turned 9 full turns instead of the previous 10 needed to fully push home the rotor. The effect was to raise rpm to 974 and watts output to 281.  OK, that's a tiny gain over 275 w.  What's important though is that it proves, and calibrates, the method. Very satisfying !

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